Fly ash, filter cake and slag from conventional incinerators, apart from unburned carbon, contain heavy metal compounds and organic hydrocarbons which can be washed out and can thereby become pollutants of water and soil. Legislation throughout the world is tending to require significant reductions in the quantity of such pollutants which can be released over a relatively short time scale so as to reduce the ecological toxic potential of slag and thereby permit safe storage, disposal or reuse. More stringent demands are also being made on other solid residues (e.g., flue dust and flue gas cleaning residues), and those residues which cannot be reused are to be processable into inert residual materials.
In general, the aim of present technologies is to reduce the volume of the non-reusable constituents with a view to keeping as small as possible the unavoidable residual material dumps. Space-intensive, environmentally safe residual material dumping of highly toxic residues proves to be very expensive, particularly if it is necessary to comply with legal requirements concerning long-term safety.
In standard grate furnaces for domestic refuse, it has hitherto not been possible to burn or incinerate at a sufficiently high temperature, quite apart from undesired local heating, such that during burning there is a melting down process of the combustion slag which results in permanent binding of heavy metals and complete burning off of organic or highly toxic compounds. Experience has shown that during slag fluidization the grates tend to be stuck during combustion, or the fluidized slag flows through the gaps in the grates. In other words, neither the presently known combustion processes, nor the plants currently in operation, are suitable for such a procedure. In the few plants involving a combination of a grate furnace with a revolving cylindrical furnace, it has hitherto been impossible to melt down slag because, in the grate furnace portion, total combustion of the waste is sought and achieved, thus leaving insufficient available energy in the revolving furnace portion to melt down the incinerated waste into slag. In addition, the revolving cylindrical furnaces do not have the necessary characteristics and equipment for drawing off the molten slag. Such furnaces are only used for complete burning off of the slag. In some special refuse disposal systems, the waste materials are burned in special refuse revolving cylindrical furnaces at very high temperatures using additional energy supplied from outside sources. In these systems, the slag problem is of a minor nature, but the systems are expensive to construct and operate because of the need for added energy.